Heat treatment to reduce embrittlement of titanium alloys

ABSTRACT

A non-burning Ti-V-Cr alloy which is heat treated to decrease its susceptibility to embrittlement in gas turbine engine compressor applications. The invention heat treat cycle consists of an isothermal holding period below the alpha solvus temperature, a slow ramp down to a lower temperature, a second holding period at a lower temperature, a third ramp down to an even lower temperature, and a final holding period at the third temperature. Other suitable heat treat cycles within the concept of the invention include a single holding period below the alpha solvus temperature double holding periods below the alpha solvus temperature with a ramp from a higher to a lower temperature and a continuous ramp below the alpha solvus temperature with no holding period.

The Government has rights in this invention, pursuant to Contract No.F33657-83-C-0092 awarded by the Department of the Air Force.

TECHNICAL FIELD

The present invention relates to the heat treatment of titanium alloys,and more specifically to a heat treatment of non-burning Ti-V-Cr alloyswhich permits an increase in the operating temperature withoutembrittlement of the alloy.

BACKGROUND ART

Pure titanium exists in the alpha crystalline form at room temperature,but transforms to the beta crystalline form at 1621° F. (883° C.).Various alloying elements increase the stability of the beta phase atlower temperatures. Certain known titanium alloys contain sufficientamounts of the beta phase stabilizers that they are largely beta phaseunder most temperature conditions and are referred to as beta titaniumalloys. The subject of these prior "beta" titanium alloys is discussedin "The Beta Titanium Alloys," by F. H. Froes et al., Journal of Metals,1985, pp. 28,37.

Titanium alloys possess an ideal combination of strength and low densityfor many aerospace applications, including gas turbine engines, andparticularly gas turbine engine compressor blades, vanes and relatedhardware. However, titanium is a highly reactive metal and can undergosustained combustion under conditions encountered in gas turbine enginecompressors. In such compressors, ambient air is compressed attemperatures on the order of 850° F. (454° C.) to pressures which may beon the order of 400 psi. The air can flow at 450 feet per second as itpasses through the compressor. Under these conditions common commercialtitanium alloys will burn uncontrollably if ignited. Ignition can occurby friction arising from the ingestion of foreign objects or as a resultof mechanical failures which cause contact between moving blades andstationary objects, at least one of which is made of titanium alloy,with friction between two titanium components being particularlytroublesome. Such combustion is a great concern to gas turbine enginedesigners who have gone to great lengths to guard against rubbingbetween titanium components.

A class of true beta titanium alloys based on compositions oftitanium-vanadium-chromium which occur in the titanium-vanadium-chromiumphase diagram bounded by the points Ti-22V-36Cr, Ti-40V13Cr andTi-22V-13Cr (all percentages herein being weight percent unlessotherwise noted) has been shown to possess a high degree of resistanceto burning (referred to hereinafter as non-burning) under the operatingconditions in a gas turbine engine. These alloys also exhibit creepstrengths which are greater than those exhibited by the strongestcommercial alloys (i.e., Ti-6-2-4-2) at elevated temperatures. A varietyof quaternary (and higher) alloying elements may be added to the basiccomposition to modify the alloy properties.

A particular titanium base alloy, having a nominal composition of 35% V,15% Cr, balance Ti, has been historically used for gas turbineapplications in the fully solutioned (all beta) condition. Whenoperating above 850° F. (454° C.) for extended periods of time, alphaphase precipitates as an essentially continuous film in the grainboundaries and embrittles the alloy, thus shortening its usefullifetime.

What is needed is a non-burning titanium alloy which can operate forextended periods of time at elevated temperatures without becomingembrittled.

What is further needed is a method of heat treating a non-burningtitanium alloy so as to render it resistant to the embrittling effectsof long term exposure at elevated temperatures.

DISCLOSURE OF INVENTION

The non-embrittling material of the present invention comprises anon-burning titanium-vanadium-chromium alloy with a composition definedby the region designated in FIG. 1 whereby the alloy is heat treated torender it resistant to precipitation of detrimental particles undernormal gas turbine engine operating conditions.

The process of the present invention comprises an initial step ofheating the material above the alpha solvus temperature for a timesufficient to produce an all beta structure, followed by heat treatingbelow the alpha solvus temperature to produce a precipitate consistingof coarse, stable alpha phase particles generally situated in the grainboundaries.

The initial heat treat step consists of holding the material at about50° F. (28° C.) above the alpha solvus temperature for from about one toabout ten hours, with about one hour generally preferred.

The sub-alpha solvus temperature heat treatment may be either isothermalor ramped. The isothermal heat treatment is conducted at a temperatureabout 150° F. below the solvus temperature for about two hours, andproduces a coarse, stable precipitate of alpha phase, which is a form ofTiO₂.

The most preferred ramp heat treatment generally consists of holding ata first temperature below the alpha solvus for a period of time, coolingat a fairly slow rate to a second, lower temperature, holding for aperiod of time at the second temperature, cooling to a still lower thirdtemperature, holding for a period of time at the third temperature, andcooling to room temperature. The ramp heat treatment initially producesa coarse precipitate of alpha phase, which is further coarsened duringthe ramp and hold portions of the cycle.

While the preferred ramp heat treatment uses three successively lowersub-solvus holding temperatures, the invention process can also becarried out effectively with more or fewer holding periods, or with aramp from a first sub-solvus temperature down to a second lowertemperature without any intermediate holding periods. While longer totalexposure times in the 1000°-1300° F. (538°-704° C.) temperature rangewould tend to improve the properties of the material, an optimum cyclemust also consider the overall cost of the operation.

These, and other features and advantages of the invention, will beapparent from the description of the Best Mode, read in conjunction withthe drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an isothermal section of the Ti-V-Cr phase diagram showing thegeneral composition region of the non-burning alloys of this invention.

FIG. 2 is a photomicrograph showing the microstructure of PWA 1274 inthe as-solutioned condition.

FIG. 3 is a photomicrograph showing the as solutioned PWA 1274 materialafter 500 hours at 1000° F. in air.

FIG. 4 is a photomicrograph of PWA 1274 processed according to theinvention.

FIG. 5 is a graph showing the results of room temperature elongationtesting of PWA 1274 after various heat treat cycles according to theinvention process followed by exposure at 1000° F. (538° C.) for 0-500hours.

BEST MODE FOR CARRYING OUT THE INVENTION

A titanium base alloy containing 35% V, 15% Cr, which lies within thecomposition ranges of a non-burning alloy as illustrated in FIG. 1, andwhich is hereinafter referred to as PWA 1274, has been shown to behighly burn-resistant in gas turbine engine compressor applications. Itis commonly used in the solutioned condition, and has a microstructureas shown in FIG. 2. The solutioning process is performed at about 1500°F. (816° C.) , approximately 50° F. (28° C.) above the alpha solvustemperature of 1450° F. (788° C.), for about one hour.

While operating above 850° F. (454° C.) for extended periods of time,the precipitation of alpha phase as a film in the grain boundariesdecreases the ductility of the alloy drastically. As measured at roomtemperature, the elongation of fully solutioned material decreases froman initial value of about 20% to about 2% after exposure in air at 1000°F. (538° C.) for 500 hours. The effect of this extended exposure on themicrostructure of the alloy is shown in FIG. 3.

By heat treating the solutioned, essentially all beta phase, materialbelow the alpha solvus temperature, but at a temperature higher than thenormal use temperature, the alpha phase, which is a form of TiO₂, iscaused to precipitate in the grain boundaries as coarse, stableparticles. These alpha particles are much less harmful to the materialthan the grain boundary films discussed above. FIG. 4 shows a typicalmicrostructure of this heat treated material.

The heat treat cycle of the invention requires that the material be inthe fully solutioned condition. The isothermal sub-solvus treatmentinvolves heating the material at a temperature about 150° F. (83° C.)below the alpha solvus temperature. The solvus temperature is stronglydependent on the oxygen content of the material, so the solvustemperature must generally be determined in order to establish the heattreat temperature for the sub-solvus step. The time required is betweenabout one-half and about ten hours, with about two hours being generallypreferred. The cooling rate from the sub-solvus treatment temperature toroom temperature should be at least 100° F. (56° C.) per hour to avoidgrain boundary precipitation.

The most preferred embodiment of the ramp heat treatment processincludes heating isothermally at a temperature about 150° F. (83° C.)below the solvus temperature for a period of about one to ten hours,with the preferred time being about two hours, cooling at a rate ofabout 25°-100° F. (14°-56° C.) per hour, with a preferred rate of about75° F. (42° C.) per hour, to a temperature about 100° F. (55° C.) belowthe first temperature, holding at the second temperature for a period ofabout one to about ten hours, preferably about six hours, cooling to athird temperature about 200° F. (111° C.) below the second temperatureand holding for a period of about one to about ten hours, preferablyabout six hours, and cooling to room temperature.

FIG. 5 is a graph showing the results of ductility testing of PWA 1274which has been sub-solvus heat treated using various heat treat cyclesaccording to the present invention. The sub-solvus heat treat cyclesapplied to the solutioned material are indicated in Table I.

                                      TABLE I                                     __________________________________________________________________________    A  1300° F. (704° C,)/2 hr, cool at 75° F.                  (42° C.)/hr to 1200 F. (649° C.)/6 hr,                          cool at 75F (42)/hr to 1000 F./6 hr, cool at > 100 F./hr to room              temperature.                                                               B  1300 F. (704° C.)/2 hr, cool at 25 F. (14° C.)/hr to            1050 F. (566° C.)/1 hr,                                                cool at > 100 F. (56° C.)/hr to room temp.                          C  1300 F. (704° C.)/2 hr, cool at 25 F. (14° C.)/hr to            1150 F. (621° C.)/1 hr,                                                cool at > 100 F. (56° C.)/hr to room temp.                          D  1300 F. (704° C.)/hr, cool at > 100 F. (56° C.)/hr to           room temperature.                                                          E  1200 F. (649° C.)/2 hr, cool at > 100 F. (56° C.)/hr to         room temperature.                                                          F  As solutioned (1500° F. or 816° C. for one                   __________________________________________________________________________       hour).                                                                 

In all cases the heat treated material showed improved ductilitycompared to the solutioned material. Even with no exposure time at 1000°F. (538° C.), the heat treated samples had better ductility than thesolutioned material. This is attributed to the fact that oxygendissolved in the beta phase is caused to migrate to the grain boundariesduring the heat treat cycle, where it precipitates as alpha phase, orTiO₂, particles. The decrease in dissolved oxygen content in the betaphase increases the ductility of the alloy.

While the measured ductility of the solution heat treated materialdecreased to about 2% after 500 hours at 1000° F. (538° C.), theductility for the sub-solvus isothermally heat treated materialsdecreased to about 5% after the same exposure. This indicates that thebenefits attributed to controlled removal of dissolved oxygen from thebeta phase are significant.

The application of a ramp heat treat cycle to solution heat treatedmaterial prior to exposure to elevated temperatures improved theductility to an even greater extent. The additional time attributed tothe ramp cycle and the second holding period apparently allowed agreater portion of the dissolved oxygen to migrate to the grainboundaries.

The ramp treatment to 1150° F. (621° C.) results in an elongation ofabout 8.5% after 500 hours at 1000° F. (538° C.), which is a significantimprovement over the elongation after exposure of the isothermally heattreated material to the same conditions. The ramp treatment to 1050° F.(566° C.) results in an elongation of about 11.5% after the sameelevated temperature exposure, which is an improvement of about 30% overthe elongation of the 1150° F. (621° C.) ramp heat treated material.This improvement is attributed to the additional time at the heat treattemperatures, since four more hours were required in the ramp portion ofthe cycle to cool down to 1050° F. (566° C.) (at 25° F., or 14° C., perhour) than were required to cool to 1150° F. (621° C.).

The three-step ramp heat treatment involved holding at 1300° F. (704°C.) for two hours, cooling at 75° F. (42° C.) to 1200° F. (649° C.),holding for six hours, cooling at 75° F. (42° C.) to 1000° F. (538° C.),holding for six hours and cooling to room temperature. As shown in FIG.5, the elongation of this material was about 15% after 500 hours at1000° F. (538° C.), which is an improvement over the two-step process.The greater exposure of the material to the elevated temperatures of theheat treat cycles during the three-step process seems to account for theincrease in measured ductility.

Although this invention has been shown and described with respect todetailed embodiments thereof, it will be understood by those skilled inthe art that various changes, omissions and additions in form and detailthereof may be made without departing from the spirit and scope of theclaimed invention.

We claim:
 1. A method to improve the high temperature stability andembrittlement resistance of a true beta titanium alloy based on titaniumand containing a nominal composition of 35% vanadium and about 15%chromium and having an alpha solvus temperature, the improvementcomprisinga. heating the alloy above the alpha solvus temperature for aperiod of time sufficient to solutionize any alpha phase present, toproduce a fully beta phase microstructure; b. heating the alloy at atemperature about 150° C. below the alpha solvus temperature and holdingfor a period of time; c. cooling at a controlled rate, whereby a smallquantity of the alpha phase is caused to precipitate and largeprecipitates rather than a continuous grain boundary film.
 2. A methodas in claim 1 wherein the alpha solvus temperature is about 788° C.
 3. Amethod as in claim 1 wherein the alloy is held at a temperature about150° C. below the alpha solvus for a period of time between about 0.5hours and 10 hours.
 4. A method as in claim 1 wherein the alloy iscooled from the sub-solvus heat treatment temperature of about 150° C.below the alpha solvus temperature, at a slow rate to at least one lowertemperature and held at this at least one lower temperature for a periodof at least one hour.
 5. A method as in claim 1 wherein, aftersubsequent exposure at a temperature of about 538° C. for about 500hours the heat treated alloy exhibits a tensile ductility of at leastabout 5% when measured at room temperature.